1. Field of the Invention
The present invention relates to a method and apparatus for forming a thin film on a substrate, and more particularly to a method and apparatus for forming a desired thin film on a substrate by using a chemical vapor deposition (i.e., CVD) process.
2. Description of the Related Art
In manufacturing a semiconductor device typified by LSI devices (i.e., Large Scale Integrated circuits), memories, microprocessors and like devices, it is necessary to form various types of thin films on the same substrate of a semiconductor. In general, these thin films comprise various types of insulation films and conductive films. The insulation films comprise: a silicon nitride (i.e., Si.sub.3 N.sub.4) film serving as an oxidation-resistant mask film used in forming a dielectric isolation film in an LSI device of MOS (i.e., Metal Oxide Semiconductor) type; and, an insulation film formed of a silicon oxide film, which serves as a surface protection film or the like. On the other hand, as for the above-mentioned conductive films, they comprise: a polysilicon film serving as gate wirings, or the like ; and, a tungsten film serving as contact plugs or the like in realizing a so-called multilevel interconnection or metallization.
As a method for forming the above-mentioned thin films, heretofore widely used is an LPCVD (i.e., Low Pressure Chemical Vapor Deposition) process. In this LPCVD process, a reactor furnace having received a semiconductor substrate (which is a workpiece) is reduced in pressure in its atmosphere. Under such a reduced pressure in the atmosphere of the reactor furnace, a reactant gas is introduced into the reactor furnace to form a desired thin film on the workpiece or semiconductor substrate. In comparison with an NPCVD (i.e., Normal Pressure Chemical Vapor Deposition) process, the LPCVD process is superior to the NPCVD process in that: the former process is less in consumption of the reactant gas than the latter process; the former process may form the thin film at a relatively low temperature, which is lower than that used in the latter process; and, the former process is superior to the latter process in uniformity of film thickness of its product (i.e., thin film) or in covering properties of the thus formed thin film.
Further, as for an LPCVD unit used in the LPCVD process, though a horizontal type of the LPCVD unit was employed during the early stages, recently a vertical type of the LPCVD unit has been widely employed since the vertical type is improved: in easiness in control of the reactant gas in flow; in uniformity of heating the reactant gas; and, efficiency in chemical reaction of the reactant gas, in comparison with the horizontal type.
Now, a conventional method for forming a thin film will be described with reference to an example, in which a thin film is formed of a silicon nitride film serving as a major insulation film in the semiconductor device.
First, the LPCVD unit provided with a vertical type of a reactor furnace is arranged, wherein its reactor tube is made of quartz (i.e., SiO.sub.2). In the LPCVD process, the interior of this reactor tube is heated, and kept at a temperature of approximately 760.degree. C., which is equal to a film-forming temperature of the silicon nitride film. Then, a jig carrying thereon a set of semiconductor substrates (which are workpieces) on each of which a thin film should be formed is loaded into the reactor furnace. After that, a plurality of reactant gases, for example such as dichlorosilane (i.e., SiH.sub.2 Cl.sub.2) gas and ammonia (i.e., NH.sub.3) gas are introduced into the reactor tube to react with each other, so that a thin film, i.e., silicon nitride film is formed on each of the semiconductor substrates. Such film-forming process for forming the silicon nitride film on each of the semiconductor substrates is performed for a predetermined period of time, so that a desired silicon nitride film with a necessary film thickness is formed on each of the semiconductor substrates. After completion of formation of the desired silicon nitride film on each of the semiconductor substrates, the supply of the reactant gases is stopped to take out the jig from the reactor furnace. After the jig is taken out of the reactor furnace, then, the whole cycle in the above film-forming process is repeated with respect to a next new set of semiconductor substrates, which are carried out on another jig or the jig previously used and are loaded into the reactor tube together with the jig.
On the other hand, in the film-forming process described above, the silicon nitride film is formed not only on the surface of each of the semiconductor substrates but also on the surfaces of other members disposed inside the reactor furnace, for example such as an inner wall of the reactor tube, jig, and like members all disposed inside the reactor tube. The silicon nitride film formed on each of these members other than the semiconductor substrates forms an unnecessary thin film, formation of which is inevitable in any reactor furnace. Further, such unnecessary thin film or unnecessary silicon nitride film is accumulated to increase its film thickness particularly in the inner wall of the reactor tube when a plurality of the film-forming processes are performed in the same reactor tube. The unnecessary silicon nitride film thus accumulated on the reactor tube made of quartz differs in coefficient of thermal expansion from its substrate made of quartz, and is therefore subjected to stress due to the presence of a difference in thermal expansion coefficient, wherein the stress gradually increases as the film thickness of the unnecessary silicon nitride film increases due to its accumulation through the plurality of the film-forming processes.
FIG. 17 shows a longitudinal sectional view of an essential part of the reactor furnace, illustrating the above phenomenon. In FIG. 17, for example, when the accumulated film thickness of the unnecessary silicon nitride film 53 formed on each of an outer tube 51, inner tube 52 and like members disposed inside the reactor furnace exceeds a predetermined value, the unnecessary silicon nitride film 53 cracks of its self due to its own stress to produce a crack 54 together with contaminant particles 55 (i.e., fragments of the unnecessary silicon nitride film 53 itself), as shown in FIG. 18. These contaminant particles 55 thus produced are naturally scattered, and fall on the surface of each of the semiconductor substrates to cause each of the semiconductor substrates to suffer from contamination (i.e., fall-on defects deposited on each of the semiconductor substrates). FIG. 19 is a plan view of the surface of the semiconductor substrate 56 formed by the conventional method for forming the thin film, illustrating the contaminant particles 55 which are scattered from the unnecessary silicon nitride film 53 and now deposited on the surface of the semiconductor substrate. Each of the contaminant particles 55 shown in FIG. 19 has a diameter of equal to or more than 200 nm. As is clear from FIG. 19, the contaminant particles 55 thus scattered and deposited on the surface of the semiconductor substrate 56 are concentrated in a peripheral area of the semiconductor substrate 56. This is because the peripheral area of the semiconductor substrate 56 is disposed in the vicinity of the inner tube 52 of the reactor furnace.
FIG. 20 is a graph showing the relationship between: the batch process numbers (in the x-axis, i.e., abscissa); and, each of the number of contaminant particles (in the left-hand y-axis, i.e., ordinate) and the accumulated film thickness of the unnecessary thin film (in the right-hand y-axis, i.e., ordinate), according to the conventional method for forming the thin films.
In FIG. 20: the reference letter "A" denotes the accumulated film thickness of the unnecessary thin film; "B" denotes the number of the contaminant particles; and, the reference letters "a", "b" and "c" denote an upper (i.e., top), an intermediate (i.e., center) and a lower area of the reactor furnace, respectively.
As is clear from FIG. 20, the accumulated film thickness of the unnecessary thin film linearly increases in proportion to the batch process numbers. Further, the number of the contaminant particles falling on the surface of the semiconductor substrate steeply increases in the 7th batch process. This tendency is true not only in this 7th batch process but also in the other batch processes subsequent to the 7th batch process. This is because, as described above, the accumulated film thickness of the unnecessary silicon nitride film reaches a predetermined value so that the stress produced in the unnecessary silicon nitride film exceeds a critical point.
As a result, due to the presence of the contaminant particles causing the fall-on defects of the semiconductor substrate, a desired silicon nitride film deposited on the semiconductor substrate is impaired in quality. Further, in maintenance which is heretofore done in the prior art, when the number of the contaminant particles increases, the reactor furnace is disassembled to do the maintenance, in which the furnace's constituent elements such as the outer tube, inner tube and the other elements are subjected to washing treatments. Consequently, in the prior art, it is necessary to do the maintenance at relatively short time intervals. In general, doing such maintenance (i.e., a series of maintenance jobs) takes substantially a whole day, during which it is necessary for the reactor furnace to be out of action. Thus, it is apparent that a need exists in the art for doing the maintenance jobs at much more prolonged time intervals than that of the prior art.
A thin film forming apparatus, in which the contaminant particles of the above-mentioned unnecessary silicon nitride film thus formed are prevented from affecting the properties of a desired thin film formed on the substrate, is disclosed in the prior art, for example such as Japanese Laid-Open Patent Application No. Hei 7-263370, in which: production of the contaminant particles is prevented by fabricating both the reactor tube and the jig for carrying thereon the workpieces (i.e., substrates) from a material having the same thermal expansion coefficient as that of the unnecessary silicon nitride film.
However, in a method for forming a desired thin film by using the above-mentioned film forming apparatus, it is necessary to arrange both the reactor tube and the jig each made of the material having the same thermal expansion coefficient as that of the unnecessary silicon nitride film, which raises the manufacturing cost due to the necessity of employing a special material in each of the reactor tube and the jig. Further, even when both the reactor tube and the jig for carrying thereon the substrates are made of the material having the same thermal expansion coefficient as that of the unnecessary silicon nitride film, it is recognized that a large number of contaminant particles are produced when the film thickness of the unnecessary silicon nitride film thus formed inside the reactor furnace exceeds approximately 2500 nm, which is slightly larger than the film thickness of the unnecessary silicon nitride film formed in the prior art.
Further, another method for forming the desired thin film without producing the contaminant particles is disclosed in the prior art, for example such as Japanese Laid-Open Patent Application No. Hei8-45859, in which: various types of desired thin films are formed by loading the workpiece (i.e., semiconductor substrate) into the reactor tube at a velocity of equal to or less than 20 mm/min, wherein the reactor tube is kept at a temperature of equal to or less than 60.degree. C.
In the conventional method disclosed in the Japanese Laid-Open Patent Application No.Hei8-45859, only described are the conditions in loading the semiconductor substrate into the reactor tube. In other words, there is no description as to means for preventing the contaminant particles from being produced when the unnecessary thin film is formed inside the reactor tube in a condition in which the semiconductor substrate has been loaded into the reactor tube. Consequently, also in this conventional method, the problem of the production of contaminant particles in forming the desired thin film remains unsolved.